Engine whose fuel is a product other than a petroleum product

ABSTRACT

The invention relates to an injection internal combustion engine. 
     The engine has a device for injecting fuel, preferably at high pressure, into the cylinder of cylinders, a device adjusting the injection advance in dependence on the load, and a controlled ignition device, the said engine being arranged to use a fuel other than a petroleum product, preferably an alcohol based product, and being characterized in that its dimensional characteristics are determined to supply a volumetric compression ratio between 12 and 20, preferably between 15 and 18 and in particular close to 16, and in that the device adjusting the injection advance is such that it supplies, at least for starting, a high value of advance, particularly of the order of 180°.

This is a division of application Ser. No. 775,039 filed Mar. 7, 1977,now U.S. Pat. No. 4,123,997.

The present invention relates to an engine utilising a fuel which is aproduct other than a petroleum product, particularly an alcohol, such asmethanol. An engine of this kind permits almost complete elimination ofpolluting substances in the exhaust gas emission.

The invention relates more particularly to a piston engine fed by acarburettor, a low pressure (LP) injector, or a high pressure (HP)injection pump associated with each cylinder, the said motor beingcapable of burning various fuels comprising products other thanpetroleum products, and particularly fuel having a high heat ofvaporization, for example pure or denatured methanol alone or mixed withconventional petroleum fuels or with other lower aliphatic alcohols.

It is well known that increasing use is being made of motorised vehiclesor of equipment having an engine, which are generally provided with anengine fed with petroleum products. An engine of this kind belongs tothe class of internal combustion engines, whatever the cycle thereof,this class comprising controlled ignition carburettor engines andself-ignition diesel engines. Such engines increase pollution of theatmosphere, because their exhaust gases contain toxic substances, suchas carbon monoxide and oxides of nitrogen. Numerous techniques have beenapplied in the attempt to reduce the production of polluting substancesin the exhaust gases by making modifications to existing engines, buthave not been successful in achieving complete elimination of thesetoxic substances. Furthermore, they often reduce the thermal efficiencyof the engine.

Tests have also been made in burning in heat engines fuels other thanpetroleum products, for example fuels based on alcohols, but hithertoefficiency has not been satisfactory and the operation of the engine,particularly as regards starting from cold, has been problematical.

The invention proposes an engine consuming fuels comprising productsother than petroleum products and having excellent thermal efficiency;this results in the absence of unburned material and almost completeelimination of toxic or corrosive gaseous constituents in the engineexhaust gases when fuels having a high heat of vaporization are burnedin this engine.

At the present time the two main categories of engines used arecontrolled ignition (petrol) engines and self-ignition (diesel) engines.

The petrol (controlled ignition) engine has an external carburationdevice, such as a carburettor, or a low or medium pressure injectionpump for preparing the carburetted mixture externally of the cylinders.An engine of this kind is provided with a controlled ignition device,for example comprising sparking plugs, and its volumetric compressionratio is normally between about 7 and 9. In the construction of theseengines up to the present time volumetric compression ratios of 10 havenever been exceeded, while ratios of about 10 have been achieved onlyexperimentally. Petrol, even of the best quality with the highest octanenumber (commercial), will in fact accept so high a ratio as 10 only ifthe actual ratio is low and the apparent (or geometric) ratio is high.This is the case with very advanced engines rotating at very highspeeds.

In a diesel engine, air is mixed with the fuel inside the cylinder withthe aid of a high pressure injection pump and with self-ignition of thefuel mixture at the end of the compression stroke. The volumetriccompression ratio varies from 14 to 23 depending on the size of theengines and their rotational speed. High speed diesel engines of theautomobile kind usually have volumetric compression ratios of about 20.

In a general aspect the invention relates to an internal combustionengine having in particular a preferably high pressure device forinjecting fuel into the cylinder or cylinders, a device for adjustingthe injection advance in dependence on the load, and a controlledignition device, the engine being arranged to use a fuel comprising aproduct other than a petroleum product, preferably based on an alcohol,and being characterized in that the device for adjusting injectionadvance is such that it supplies, at least for starting, a high value ofadvance, particularly of the order of 180°.

According to the invention the device for adjusting injection advance isdesigned to reduce the advance when the load increases.

The fuel used in the engine of the invention comprises a fuel having ahigh heat of vaporization, particularly one selected from the loweraliphatic alcohols, such as methanol, ethanol, propanol, or isopropanoland the butanols.

No engine known at the present time either of the controlled ignitiontype or of the self-ignition (diesel) type is able to burn withacceptable efficiency fuels having a high heat of vaporization.

If in fact methanol is burned, for example in an engine whose volumetriccompression ratio is equal to 10, the proportion of unburned material isexcessive and thermal efficiency is poor. If methanol is burned in adiesel engine whose ignition system has been converted to controlledignition and whose volumetric compression ratio is from 20 to 22, thisratio is too high for methanol and satisfactory thermal efficiency isnot achieved.

In addition, the combustion chamber designed for self-ignition isgenerally not utilisable for controlled ignition, which is essential forthe ignition of fuels of the alcohol type.

According to the invention the engine fed with lower aliphatic alcoholhas dimensional characteristics determined to supply a volumetriccompression ratio between 12 and 20 and preferably between 15 and 18,this ratio being sufficient for combustion to be complete. A ratio ofabout 16 corresponds to an optimisation ratio for an engine utilisingmethanol for the operating conditions generally occurring inautomobiles. In this case the overall thermal efficiency decreases bothabove and below this ratio.

When methanol is burned in an engine which is constructed in accordancewith the invention and whose volumetric compression ratio is 16, goodthermal efficiency is obtained which is constantly higher than theefficiency of the "petrol" engine and is very close to that of the"diesel" engine (about 2000 K calories per CV per hour) in theautomobile range.

In the engine according to the invention combustion with optimumefficiency is obtained without the use of a heat exchanger, andtherefore without external heating of the air, of the fuel, or of themixture.

Because of the high heat of vaporization of the fuel used in the engineaccording to the invention, considerable cooling occurs at the moment ofthe atomization of the fuel, or at the moment when the carburettedmixture is formed. During the travel of the carburetted mixture it maytherefore be advantageous to attempt to achieve the longest heatexchange time between the fluids constituting the mixture. Furthermore,it is important that before the ignition the carburetted mixture shouldattain an adequate end of compression temperature to make it possible toachieve easy ignition of the carburetted mixture, particularly in orderto ensure normal starting of the engine and good running at low load.This end of compression temperature depends on the compression ratio, onthe characteristics of the fuel used (heat of vaporization), and on themethod of feeding the engine.

The feeding of the engine is normally achieved by one of the followingthree means:

(1) Conventional carburettor: The primary calories are supplied by theheat of the ambient air drawn in and by the progressive heating of thewalls of the carburettor and of the piping, resulting from the normalheating of the engine. At the end of a certain period of operation thesuction temperature finally becomes relatively high in relation to theatmosphere.

(2) "Low pressure" injection: A variant of conventional carburation inwhich it is advantageous to apply multi-cylinder feeding.

(3) "High pressure" injection: This is the arrangement that will bepreferred, without the other methods of feeding being rejected. Thismethod of feeding provides the advantage of giving a precise injectionpoint in the cycle as well as precise metering of the fuel. It is alsothe method of injection which makes it possible to achieve optimuminjection advance in conformity with the time necessary for obtainingthe best temperature exchange. It is the injection method which makes itpossible to obtain the best efficiency, because finally, due to theinjection advance, it makes it possible for the cooling effect andtherefore the calorie exchange time to be perfectly corrected. For goodefficiency of an engine burning methanol with a volumetric compressionratio of 16 the injection advance may attain 180°, that is to say half arevolution, before ignition, for the purpose of starting and at very lowloads; on the other hand, for higher loads and engine speeds thisinjection advance will be automatically reduced by a conventionalcentrifugal device, which however provides a variation towards delay.

Depending on the type of operation desired, that is to say an automobiletype engine, a highly developed type of engine, or a slow type for fixedor marine installations, of the two-stroke or four-stroke type, theinvention proposes a means of determining the characteristics of theengines to be constructed. Other characteristics and advantages will beseen from the description given below of various embodiments permittingthe utilisation of the process of the invention, this description beinggiven by way of example without limitation and with reference to theaccompanying drawings, in which:

FIG. 1 is a graph showing mean curves of compression ratios for variousfuels, plotted against the factor C defined hereinbelow:

FIG. 2 is a diagram of a combustion chamber of an engine according tothe invention;

FIG. 3 is a diagram of a modified combustion chamber of an engineaccording to the invention;

FIG. 4 is a view in cross-section corresponding to the chamber of FIG.3;

FIG. 5 is a diagram of a combustion chamber of lenticular shapeaccording to the invention;

FIG. 6 is a view in cross-section corresponding to the chamber of FIG.5;

FIG. 7 is a graph similar to the graph in FIG. 1 for mixtures ofpetroleum fuel with methanol in variable percentages;

FIG. 8 shows diagrammatically a loop feed circuit for the engine of theinvention.

In the preferred embodiment of the invention the fuel is methanol.

Methanol has physicochemical properties different from those ofpetroleum products, particularly gas oil and petrols. The properties ofmethanol, which are described in detail below, entail necessaryfundamental changes to known engines in order to produce an engineproducing mechanical energy with optimum efficiency by burning methanol.

The description below is given more particularly for an engine burningmethanol, but it is quite obvious that it also covers engines burningother fuels selected from the lower aliphatic alcohols, either pure orin mixtures. In the course of the description information will be givenwhen the properties of aliphatic alcohols other than methanol will bedifferent from those of methanol.

100% pure methanol, which may or may or may not be denatured, ispreferably used. It is also possible to use methanol mixed with misciblepetroleum products, in any proportion.

Nevertheless, the octane number resulting from the mixtures varies fromabout 80 (number of the base petroleum product) to an octane numberconsiderably higher than 100, as the proportion of pure methanolincreases.

Since the octane number has an influence on the fundamentalcharacteristics of the engine, when determining these characteristics atthe design stage it is therefore necessary to make a choice depending onthe fuel. Once the choice has been made and the optimised engine hasbeen built, it will no longer be possible to use a fuel having verydifferent physicochemical characteristics.

In an engine optimised for methanol it will in fact be as inappropriateto burn petrol in it as to attempt to use methanol in an existing enginedesigned for petrol; this would result in abnormally high consumption offuel.

The cetane number of methanol is practically zero in relation to that ofheavy oils, as is also the case with petrol; it is therefore necessaryto utilise controlled ignition, particularly ignition by means ofsparking plugs, because there is no self-ignition at the compressionratios of 16 which are used and which are recommended for maximumefficiency.

A test has been made with a compression ratio of 22 and no self-ignitionoccurred; controlled ignition therefore becomes obligatory.

The threshold of the heat of vaporization of methanol is much higherthan that of petrol, which is one of the causes of the differentconstruction of alcohol engines and engines for petroleum products.

The heat of vaporization is in effect 80 calories per gram for petrol,164 calories per gram for propyl alcohol, 204 calories per gram forethanol; and 284 calories per gram for methanol. The orders of magnitudeare therefore 80 on the one hand and 200 on the other hand, thusentailing considerable differences between the characteristics of anengine according to the invention and engines already in use.

The engine of the invention is suitable for propyl alcohol, that is tosay for values ranging from 150 calories per gram and considerablybeyond 284 calories per gram, the latter value corresponding to the heatof vaporization of methanol. As previously indicated, it should be notedthat starting from a certain latent heat of vaporization the adaptationsmade for alcohols will no longer permit the use of petrols.

The high value of the heat of vaporization of the fuel used according tothe invention results in considerable cooling at the moment when thefuel is atomized or at the moment when the carburetted mixture isformed. Apart from the high compression ratio, provision is made tocorrect the cooling effect either by maximum elongation of the heatexchange time (carburettor, LP injection), or by a large injectionadvance in the case of HP injection. In the latter case, by adjustingthe value of the admission advance of the fuel it has been found intests that it is possible to operate with methanol with compressionratios of the diesel type of the order of 20 simply by increasing thevalue of the injection advance in comparison with that for dieselengines. In this case it is obvious that the method of ignition must bechanged by providing the diesel engine with controlled ignition.Nevertheless, in order to obtain an engine which is more flexible andpermits heavier overloads, the ratio of 16 appeared to be an optimum,and in this case and for the direct high pressure method of injectionthe injection advance is of the order of 180°, that is to say onehalf-revolution before ignition.

The volumetric compression ratio must be selected at about 16 and moregenerally between 15 and 18, in order to take into account the methodsof fuel supply (preferably 15 for a carburettor and preferably 17 to 18for HP injection) and in order to obtain maximum efficiency in aconventional automobile engine with controlled ignition and fed withmethanol, in comparison with engines of the same class fed with petroland using a compression ratio of the order of 7 to 9.

For another engine, for example a slow engine running at 1000revolutions per minute, and generally heavier, the optimum ratio may be12 with methanol.

In the designing of the engine according to the invention it appearsuseful to study the coefficients or indices of similitude betweenengines in dependence on the fuels used.

By way of example a first index C has been defined, which is alwayslower than 1 and which is the ratio C_(a) /C_(g) of the value of theactive stroke C_(a) of the piston in the cylinder during the compressionphase to the value of the geometric or constructive stroke C_(g) of thepiston in the cylinder.

This index C constitutes a characteristic of an engine which may beeither a 2-stroke or 4-stroke type, and which may be a slow engine or ahigh speed engine, an automobile engine, or a motorcycle engine.

This index varies from 0.5 to 1. It is 0.95 for a heavier slow engine;0.80 for a conventional automobile engine; 0.70 for a highly developedvery high speed engine.

By introducing the notion of different fuels, it is possible toestablish the following table of the values of C and of compressionratios for fuels having a high octane number, such as (ordinary andsuper) petrol and methanol. For heavy oils it would be possible to plota third series of curves, in which case however the cetane number wouldbe introduced.

    ______________________________________                                        Optimum compression ratio (mean values)                                       C       Super petrol    Pure methanol                                         ______________________________________                                        0.95    5 to 6          12 (11 to 13)                                         0.80    7 to 9          16 (15 to 18)                                         0.70    9 to 10         18 (17 to 20)                                         0.60    12.5            20 (19 to 22)                                         ______________________________________                                    

FIG. 1 shows the mean curves of the compression ratios τ (on theabscissa) plotted against Ca/Cg=C (on the ordinate for various fuels).

These curves have been plotted from measurements made in differentengines fed with high octane number fuels, with super petrol for curve Aand with methanol for curve B.

The value C=0.95 corresponds to a slow four-stroke engine; C=0.70corresponds to a highly developed four-stroke engine, and the last valueof C=0.60 corresponds to a motorcycle equipped with a 19 HP, 8800 rpmSACHS engine. The figures shown in the Table were all taken from anexperimental model fed with these two fuels.

The hatched zone containing the curve B corresponds to the zone ofoptimum efficiency of an engine fed with methanol; the correspondingvalues on the abscissa are therefore the most expedient compressionratios.

A (hatched) zone has been defined between two end curves, because for adetermined value of C the compression ratio varies, for optimumefficiency, in dependence on the method of ignition of the carburettedmixture. Thus, for a four-stroke engine with controlled ignition havinga ratio C=0.80, that is to say for a conventional automobile engine, themean compression ratio will be 16 with methanol; it will preferably be15 with a carburettor and 17 with a high pressure injection pump. Thiscan be seen from the curves in broken lines plotted between the twolimit curves. Under these conditions the theoretical efficiency will beoptimum. For a selected fuel these considerations thus make it possibleto determine for a given C the best compression ratio corresponding to adetermined method of ignition.

A comparative study of engines in dependence on the fuels used leads totaking into account a factor θ_(f) which is of great importance, θ_(f)being the final temperature obtained in the cylinder at the end of thecompression of the carburetted mixture. It has been indicated that thistemperature depends on the method of feeding the engine (temperature ofair drawn in), of the compression ratio which in turn depends on C, andon the fuel used (heat of vaporization). It should be observed that fora determined fuel this necessary temperature θ_(f) is a constant, havingregard to the physicochemical properties of the fuel and in particularto the latent heat of vaporization. This has the consequence that if theair drawn in remains at a temperature which, for example, is close toatmospheric temperature the compression ratio is automaticallydetermined in accordance with the value of the heat of vaporization ofthe fuel, and consequently the ratio C, in dependence on the type ofengine selected--a slow engine, conventional automobile engine, or highperformance engine.

This temperature θ_(f) must be controlled at every moment and must notexceed the ignition temperature θ_(i) of the fuel, failing which, if thecompression ratio remains constant, there is a risk of harmful sideeffects on thermal efficiency (self-ignition, knocking, etc.).

The same is true of starting from cold. If it is desired that the engineshould start in cold weather on the first rotation of the crankshaft, itis necessary that from the outset this temperature θ_(f) should be closeto θ_(i), failing which the engine will not start. The engine does notmake provision for the utilisation of previous external heating of theair or fuel, or of the carburetted mixture, before introduction into thecylinder, with the aid of a heat exchanger, because at the time whenthese calories are required they are not available.

On the contrary, making use of compression only, the value of theheating is known exactly. Furthermore, in this last case, the mixtureintroduced in the cold state into the cylinder and then compressed isconsiderably more dense than when introduced in the hot state into thecylinder.

The physicochemical characteristics of a fuel according to the inventionnecessitates substantial differences between an engine burning thesefuels and an engine fed with petroleum products.

The dimensioning of the engine must be calculated in dependence on therequirements which have to be taken into account in order to obtain goodefficiency, that is to say a high compression ratio, controlled ignitionand, in the case of high pressure injection, injection advance greaterthan that utilised in a diesel engine, and in the case of the othermethods of feeding an ignition advance greater than that which isutilised for petrol.

With regard to the combustion chambers, two completely different classesof chamber already exist, depending on whether the fuels have a highoctane number or a high cetane number. With methanol it is necessary toconsider a new class of combustion chamber which takes into account ahigh compression ratio, the speed of flame propagation, which is slower,and also ignition times which are a little longer.

In combustion chambers which have been designed for methanol it has beenattempted to obtain chambers of revolution having a semi-lenticular orlenticular shape, with an ignition device comprising, for example, asparking plug whose position is central, along the axis, or parallel tothe axis of the cylinder, and whose inclination is preferably below 30°.

In FIG. 2 a combustion chamber having a semi-lenticular shape is shown.The piston 1 and a cylinder head 2 are shown diagrammatically. Thiscylinder head is substantially flat, while the piston has a bowl-shapedcavity 3. In the example illustrated the diameter of the bowl is of theorder of two-thirds of the bore, and the depth of the bowl (around it inFIG. 2) defines a combustion chamber supplying a combustion ratiodetermined in the manner described with reference to FIG. 1, dependingon the final combustion temperature and the method of feeding. In theexample illustrated the sparking plug 4 is disposed on the axis ofsymmetry of the combustion chamber, but it could also be offset in thelateral direction towards one side or the other.

FIGS. 3 and 4 show a modified combustion chamber for an engine accordingto the invention, this chamber having a semi-lenticular shape as in FIG.2. In this modified embodiment the combustion chamber is formed in thecylinder head 12, while the piston 11 has a substantially flat upperface. The bowl 13 has moreover the same relative dimensions as the bowl3 in FIG. 2. In this modified embodiment the sparking plug 14 isinclined obliquely in relation to the axis of the piston and of thechamber, forming with that axis an angle ≦30°. The axis of the sparkingplug can therefore assume positions between the axes OX and OY, formingangles of 30° on each side of the axis of symmetry. FIG. 4, which is across-section corresponding to FIG. 3, shows by way of illustration theposition of the valves 15 and 16 and that of the sparking plug 14.

FIGS. 5 and 6 show a combustion chamber of lenticular shape. In thisexample the piston 21 has in its upper face an axially symmetricalcavity 23. Similarly, the cylinder head 22 has a cavity 27 disposedfacing the cavity 23. The volume represented by the cavities 23 and 27together forms a combustion chamber whose total volume must correspondto the combustion ratio. In this example the sparking plug 24 isdisposed axially.

FIG. 6, which corresponds to FIG. 5, shows the axial arrangement of thesparking plug 24 and the symmetrical arrangements in pairs of the twoadmission valves 25 and two exhaust valves 26, the trace of thelongitudinal plane of the engine being represented by the line z'--z.

The diagrammatical representations described above are sufficient toenable those skilled in the art to understand the structure of thecombustion chambers of the engine of the invention.

The relative positions of the sparking plug and injector are notcritical, but the arrangement for obtaining optimum efficiency consistsin placing the sparking plug in the manner shown in FIG. 2, that is tosay in the axis of the combustion chamber or along a line parallel tothe axis of the cylinder, or in all cases inside a cone having a totalopening of 45° to 60° at the apex, the axis of which cone is substantialparallel to the cylinder. With regard to the injector, its position isnot at all critical; its position may be selected so that its jet ofatomized fuel does not arrive so as to cool directly the sparking plugor ignition device.

The fact that the sparking plug or the ignition system is placed in theaxis of the combustion chamber, or substantially parallel thereto,entails positioning of the valves 15 and 16 (in a 4-stroke engine) oneach side of the plug 14, in cases where the method of fuel feedutilises a carburettor. In cases where high performances are desired,and when it is necessary to increase the flow of fluids in circulation,the valves could be grouped in two pairs as shown in FIG. 6, that is tosay two parallel valves for admission and two parallel valves forexhaust.

It will be recalled that if a considerable ignition advance is providedwhen the feed means comprises a carburettor, a heavy injection advancemust be provided when injectors are used, because of the high value ofthe heat of vaporization. For an engine whose compression ratio is 16 aninjection advance is provided which may amount to half a revolutionbefore ignition, thus permitting starting from cold and operation withlow loads and at low engine speeds. If the injection advance or ignitionadvance remains at the value adopted for petrol, a very marked fall ofpower and performance is found, because the delay in the ignition of theair-methanol mixture is longer and the flame propagation speed slightlylower. The power-to-weight ratio or weight-to-power ratio is increasedby from 30 to 35%, acceleration is substantially more powerful,operation much more flexible, and great ability to take overloads,particularly at low engine speeds, is found.

The efficiency of the engine of the invention is higher than that of thepetrol engine and substantially equal to that of a diesel engine, whilebeing much less noisy than a conventional diesel engine. In addition,the exhaust gases of this engine are colourless, odourless, and giverise to very little pollution.

In the foregoing the particular case of the utilisation as fuel ofmethanol in the practically pure state has been studied. However,although it is actually possible to contemplate the utilisation of purealcohol, and in particular pure methanol, as fuel, the practicalevolution of engines towards this solution will probably be able to takeplace only progressively, having regard to the conversions to be made toconventional engines designed for petroleum fuels. It is therefore alsonecessary to consider the problem of the utilisation of alcohols mixedwith conventional petroleum fuels, both with a view to reducing theconsumption of the latter and with a view to reducing atmosphericpollution caused by exhaust gases.

The present invention therefore also has as an object the extension ofthe results obtained hereinabove to the case of the utilisation of amixture in variable portions of conventional fuel, such as petrol, withan alcohol such as methanol.

In the foregoing a graph has been used to define the means of selectingthe optimum volumetric compression ratio T to be achieved in the engine,this ratio then being used to determine the dimensional characteristicsof the engine. This optimum rate is given in dependence on a coefficientC, which is always lower than 1 and which is the ratio Ca/Cg of thevalue of the active stroke Ca of the piston in the cylinder during thecompression phase to the value of the geometric or constructive strokeCg of the piston in the cylinder.

This index C constitutes a characteristic of a motor, whether it is ofthe two-stroke or four-stroke type, whether it is a slow engine or ahighly developed engine, or an automobile or motorcycle engine. Thegraph given in FIG. 7 here also makes it possible to select the optimumcompression ratio for a motor whose coefficient C is known, independence on the composition of the fuel mixture. The curve 30corresponds to the case of a pure conventional petroleum fuel, forexample super petrol having a high octane number. The curve 31 is a meancurve which on the contrary corresponds to the case of the utilisationof pure methanol. The intermediate curves 32, 33 . . . correspond tomixtures defined by their percentage of methanol; thus, curve 30corresponds to 0% and the curves 31, 34, and 35 to 100, of methanol,while the curve 34 corresponds to feeding with a carburettor and curve35 to injection. As an example, for a value of C=0.73 there will be anoptimum compression ratio T of 9.7 for pure petrol, 16 for puremethanol, and 13 for a 50% mixture of both.

The addition, mixed with petroleum products, of a liquid containingoxygen in its chemical formula makes it possible to achieve completecombustion and improved energy efficiency, and finally reduces pollutionto a minimum.

On the other hand, however, the use of mixtures of alcohol and petroleumproduct entails subsidiary problems of its own, particularly the need toavoid the phenomena known under the names of unmixing, percolation, andvapour lock.

Unmixing is the tendency of the two liquids, alcohol and petrol, toseparate; the larger the amount of water contained in the mixture, themore pronounced this phenomenon will be.

Percolation is nascent boiling of the methanol component in the mixture,due to the fact that methanol has a rather low boiling point. Thephenomenon of vapour lock may also occur; this also occurs with petrolin very hot weather and gives rise to the blocking of the carburettorthrough the formation of a plug or block of vapour.

According to the invention these phenomena are eliminated by a devicefeeding the engine with fuel with a looped circuit, as shown in FIG. 8.

Referring to FIG. 8, the tank 40 is carefully heat-insulated to avoidsolar radiation and also reflected heat coming from road surfaces. Thefeed pump 41 is an electric pump of conventional type, operated on theclosing of a contact and disposed as close as possible to the tank 40.The flow of the pump 41 is selected to be up to three or four times theflow consumed at full load. The pump 41 delivers into the feed pipe 42,which is also heat-insulated, although the heat insulation is not shownin the diagram in the Figure.

The carburettor 43 is of the constant level type and its float chamberis provided on its side walls with a ring of holes 44 forming anoverflow, or any other system overflowing into a pipe 45 leading back tothe tank 40 by gravity. The float chamber of the carburettor 43 is alsoprovided at its base with a calibrated orifice 46 whose diameter is lessthan 1 mm; the orifice 46 is also in communication with the return pipe45. The diameter of the orifice 46 is so determined as to enable thechamber to be emptied in about five seconds when the supply 42 is shutoff.

It is seen that in normal operation the continuous circulation of thefuel and its recycling to the heat-insulated tank prevent its localheating beyond the boiling point of methanol and entail turbulence ofthe fuel mixture, thus preventing its separation. During a halt, even ifthe temperature is at its maximum under the bonnet, the fuel does notstagnate in the float chamber but escapes through the action of gravityby way of the orifice 46 to the heat-insulated tank 40; the carburettoris completely emptied, preventing any possibility of percolation andflooding. On restarting, the pump 41 starts to operate as soon as thecontact is closed, and the flow of the pump enables the carburettorfloat chamber to be filled in from one to two seconds.

What is claimed is:
 1. A fuel feed circuit for an internal combustionengine utilizing a carburettor adapted to receive as fuel a mixture invariable proportions of a conventional petroleum product and anotherproduct, preferably an alcohol based product, comprising a tank adaptedto hold the fuel, pump means for delivering the fuel through firstconduit means from said tank to said carburettor at a flow rate greaterthan the fuel consumption of the engine at full load, said carburettorincluding a fuel receiving float chamber, first means for returning fuelfrom said float chamber to said tank through second conduit meanswithout flowing through said first conduit means during normaloperations, said pump means, tank and the first and second conduit meansdefining a closed loop fluid circuit whereby fuel is continuouslycirculated through the tank, carburettor and the first and secondconduit means during normal operations whereby local heating andseparation of the fuel into its individual products is precluded, andsecond means for draining said float chamber into said second conduitmeans upon the deactivation of said pump means thereby preventingpercolation of the alcohol based product.
 2. The fuel feed circuit asdefined in claim 1 including third conduit means in part defining saidsecond means in fluid communication between said float chamber and saidsecond conduit means.
 3. The fuel feed circuit as defined in claim 2wherein said float chamber includes a side wall and a bottom wall, andfirst and second aperture means in said respective side and bottom wallsin fluid communication with said second and third conduit means,respectively.
 4. The fuel feed circuit as defined in claim 3 wherein thefuel is gravity fed from said float chamber through said second aperturemeans and third conduit means to said tank.
 5. The fuel feed circuit asdefined in claim 3 wherein said carburettor is of the constant-leveltype, and said first aperture means are a plurality of overflowapertures in said side wall.
 6. The fuel feed circuit as defined inclaim 4 wherein said carburettor is of the constant-level type, and saidfirst aperture means are a plurality of overflow apertures in said sidewall.